Network Working Group B. Adamson Request for Comments: 5740 Naval Research Laboratory Obsoletes: 3940 C. Bormann Category: Standards Track Universitaet Bremen TZI M. Handley University College London J. Macker Naval Research Laboratory November 2009 NACK-Oriented Reliable Multicast (NORM) Transport Protocol Abstract This document describes the messages and procedures of the Negative- ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol. This protocol can provide end-to-end reliable transport of bulk data objects or streams over generic IP multicast routing and forwarding services. NORM uses a selective, negative acknowledgment mechanism for transport reliability and offers additional protocol mechanisms to allow for operation with minimal a priori coordination among senders and receivers. A congestion control scheme is specified to allow the NORM protocol to fairly share available network bandwidth with other transport protocols such as Transmission Control Protocol (TCP). It is capable of operating with both reciprocal multicast routing among senders and receivers and with asymmetric connectivity (possibly a unicast return path) between the senders and receivers. The protocol offers a number of features to allow different types of applications or possibly other higher-level transport protocols to utilize its service in different ways. The protocol leverages the use of FEC-based (forward error correction) repair and other IETF Reliable Multicast Transport (RMT) building blocks in its design. This document obsoletes RFC 3940. Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction and Applicability . . . . . . . . . . . . . . . . 4 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5 1.2. NORM Data Delivery Service Model . . . . . . . . . . . . . 5 1.3. NORM Scalability . . . . . . . . . . . . . . . . . . . . . 7 1.4. Environmental Requirements and Considerations . . . . . . 8 2. Architecture Definition . . . . . . . . . . . . . . . . . . . 8 2.1. Protocol Operation Overview . . . . . . . . . . . . . . . 10 2.2. Protocol Building Blocks . . . . . . . . . . . . . . . . . 12 2.3. Design Trade-Offs . . . . . . . . . . . . . . . . . . . . 12 3. Conformance Statement . . . . . . . . . . . . . . . . . . . . 13 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 15 4.1. NORM Common Message Header and Extensions . . . . . . . . 15 4.2. Sender Messages . . . . . . . . . . . . . . . . . . . . . 18 4.2.1. NORM_DATA Message . . . . . . . . . . . . . . . . . . 18 4.2.2. NORM_INFO Message . . . . . . . . . . . . . . . . . . 28 4.2.3. NORM_CMD Messages . . . . . . . . . . . . . . . . . . 29 4.3. Receiver Messages . . . . . . . . . . . . . . . . . . . . 47 4.3.1. NORM_NACK Message . . . . . . . . . . . . . . . . . . 47 4.3.2. NORM_ACK Message . . . . . . . . . . . . . . . . . . . 53 4.4. General Purpose Messages . . . . . . . . . . . . . . . . . 55 4.4.1. NORM_REPORT Message . . . . . . . . . . . . . . . . . 55 5. Detailed Protocol Operation . . . . . . . . . . . . . . . . . 55 5.1. Sender Initialization and Transmission . . . . . . . . . . 57 5.1.1. Object Segmentation Algorithm . . . . . . . . . . . . 58
5.2. Receiver Initialization and Reception . . . . . . . . . . 59 5.3. Receiver NACK Procedure . . . . . . . . . . . . . . . . . 59 5.4. Sender NACK Processing and Response . . . . . . . . . . . 62 5.4.1. Sender Repair State Aggregation . . . . . . . . . . . 62 5.4.2. Sender FEC Repair Transmission Strategy . . . . . . . 63 5.4.3. Sender NORM_CMD(SQUELCH) Generation . . . . . . . . . 64 5.4.4. Sender NORM_CMD(REPAIR_ADV) Generation . . . . . . . . 65 5.5. Additional Protocol Mechanisms . . . . . . . . . . . . . . 65 5.5.1. Group Round-Trip Time (GRTT) Collection . . . . . . . 65 5.5.2. NORM Congestion Control Operation . . . . . . . . . . 67 5.5.3. NORM Positive Acknowledgment Procedure . . . . . . . . 75 5.5.4. Group Size Estimate . . . . . . . . . . . . . . . . . 77 6. Configurable Elements . . . . . . . . . . . . . . . . . . . . 77 7. Security Considerations . . . . . . . . . . . . . . . . . . . 80 7.1. Baseline Secure NORM Operation . . . . . . . . . . . . . . 82 7.1.1. IPsec Approach . . . . . . . . . . . . . . . . . . . . 83 7.1.2. IPsec Requirements . . . . . . . . . . . . . . . . . . 85 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 86 8.1. Explicit IANA Assignment Guidelines . . . . . . . . . . . 87 8.1.1. NORM Header Extension Types . . . . . . . . . . . . . 87 8.1.2. NORM Stream Control Codes . . . . . . . . . . . . . . 88 8.1.3. NORM_CMD Message Sub-Types . . . . . . . . . . . . . . 88 9. Suggested Use . . . . . . . . . . . . . . . . . . . . . . . . 89 10. Changes from RFC 3940 . . . . . . . . . . . . . . . . . . . . 90 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 91 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 91 12.1. Normative References . . . . . . . . . . . . . . . . . . . 91 12.2. Informative References . . . . . . . . . . . . . . . . . . 92
1. Introduction and Applicability The Negative-ACKnowledgment (NACK) Oriented Reliable Multicast (NORM) protocol can provide reliable transport of data from one or more senders to a group of receivers over an IP multicast network. The primary design goals of NORM are to provide efficient, scalable, and robust bulk data (e.g., computer files, transmission of persistent data) transfer across possibly heterogeneous IP networks and topologies. The NORM protocol design provides support for distributed multicast session participation with minimal coordination among senders and receivers. NORM allows senders and receivers to dynamically join and leave multicast sessions at will with minimal overhead for control information and timing synchronization among participants. To accommodate this capability, NORM protocol message headers contain some common information allowing receivers to easily synchronize to senders throughout the lifetime of a reliable multicast session. NORM is self-adapting to a wide range of dynamic network conditions with little or no pre-configuration. The protocol is tolerant of inaccurate timing estimations or lossy conditions that can occur in many networks including mobile and wireless. The protocol can also converge and maintain efficient operation even in situations of heavy packet loss and large queuing or transmission delays. This document obsoletes the Experimental RFC 3940 specification. This document is a product of the IETF RMT working group and follows the guidelines provided in the Author Guidelines for Reliable Multicast Transport (RMT) Building Blocks and Protocol Instantiation documents [RFC3269]. Statement of Intent This memo contains the definitions necessary to fully specify a Reliable Multicast Transport protocol in accordance with the criteria of IETF Criteria for Evaluating Reliable Multicast Transport and Application Protocols [RFC2357]. The NORM specification described in this document was previously published in the Experimental Category [RFC3940]. It was the stated intent of the RMT working group to re- submit this specifications as an IETF Proposed Standard in due course. This Proposed Standard specification is thus based on RFC 3940 and has been updated according to accumulated experience and growing protocol maturity since the publication of RFC 3940. Said experience applies both to this specification itself and to congestion control strategies related to the use of this specification. The differences between RFC 3940 and this document are listed in Section 10.
1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 1.2. NORM Data Delivery Service Model A NORM protocol instance (NormSession) is defined within the context of participants communicating connectionless (e.g., Internet Protocol (IP) or User Datagram Protocol (UDP)) packets over a network using pre-determined addresses and host port numbers. Generally, the participants exchange packets using an IP multicast group address, but unicast transport MAY also be established or applied as an adjunct to multicast delivery. In the case of multicast, the participating NormNodes will communicate using a common IP multicast group address and port number chosen via means outside the context of the given NormSession. Other existing IETF data format and protocol standards MAY be applied to describe and convey the necessary a priori information for a specific NormSession (e.g., Session Description Protocol (SDP) [RFC4566], Session Announcement Protocol (SAP) [RFC2974], etc.). The NORM protocol design is principally driven by the assumption of a single sender transmitting bulk data content to a group of receivers. However, the protocol MAY operate with multiple senders within the context of a single NormSession. In initial implementations of this protocol, it is anticipated that multiple senders will transmit independently of one another and receivers will maintain state as necessary for each sender. In future versions of NORM, it is possible some aspects of protocol operation (e.g., round-trip time collection) will provide for alternate modes allowing more efficient performance for applications requiring multiple senders. NORM provides for three types of bulk data content objects (NormObjects) to be reliably transported. These types include: 1. static computer memory data content (NORM_OBJECT_DATA type), 2. computer storage files (NORM_OBJECT_FILE type), and 3. non-finite streams of continuous data content (NORM_OBJECT_STREAM type). The distinction between NORM_OBJECT_DATA and NORM_OBJECT_FILE is simply to provide a hint to receivers in NormSessions serving multiple types of content as to what type of storage to allocate for received content (i.e., memory or file storage). Other than that
distinction, the two are identical, providing for reliable transport of finite (but potentially very large) units of content. These static data and file services are anticipated to be useful for multicast-based cache applications with the ability to reliably provide transmission of large quantities of static data. Other types of static data/file delivery services might make use of these transport object types, too. The use of the NORM_OBJECT_STREAM type is at the application's discretion and could be used to carry static data or file content also. The NORM reliable stream service opens up additional possibilities such as serialized reliable messaging or other unbounded, perhaps dynamically produced content. The NORM_OBJECT_STREAM provides for reliable transport analogous to that of the Transmission Control Protocol (TCP), although NORM receivers will be able to begin receiving stream content at any point in time. The applicability of this feature will depend upon the application. The NORM protocol also allows for a small amount of out-of-band data (sent as NORM_INFO messages) to be attached to the data content objects transmitted by the sender. This readily available out-of- band data allows multicast receivers to quickly and efficiently determine the nature of the corresponding data, file, or stream bulk content being transmitted. This allows application-level control of the receiver node's participation in the current transport activity. This also allows the protocol to be flexible with minimal pre- coordination among senders and receivers. The NORM_INFO content is atomic in that its size MUST fit into the payload portion of a single NORM message. NORM does NOT provide for global or application-level identification of data content within its message headers. Note the NORM_INFO out- of-band data mechanism can be leveraged by the application for this purpose if desired, or identification can alternatively be embedded within the data content. NORM does identify transmitted content (NormObjects) with transport identifiers that are applicable only while the sender is transmitting and/or repairing the given object. These transport data content identifiers (NormTransportIds) are assigned in a monotonically increasing fashion by each NORM sender during the course of a NormSession. Participants, including senders, in NORM protocol sessions are also identified with unique identifiers (NormNodeIds). Each sender maintains its NormTransportId assignments independently and thus individual NormObjects can be uniquely identified during transport by concatenation of the session-unique sender identifier (NormNodeId) and the assigned NormTransportId. The NormTransportIds are assigned from a large, but fixed, numeric space in increasing order and will be reassigned during long-lived sessions. The NORM protocol provides mechanisms so the sender application can terminate transmission of data content and inform the group of this in an efficient manner. Other similar protocol control
mechanisms (e.g., session termination, receiver synchronization, etc.) are specified so reliable multicast application variants can realize different, complete bulk transfer communication models to meet their goals. To summarize, the NORM protocol provides reliable transport of different types of data content (including potentially mixed types). The senders enqueue and transmit bulk content in the form of static data or files and/or non-finite, ongoing stream types. NORM senders provide for repair transmission of data and/or FEC content in response to NACK messages received from the receiver group. Mechanisms for out-of-band information and other transport control mechanisms are specified for use by applications to form complete reliable multicast solutions for different purposes. 1.3. NORM Scalability Group communication scalability requirements lead to adaptation of NACK-based protocol schemes when feedback for reliability is needed [RmComparison]. NORM is a protocol centered around the use of selective NACKs to request repairs of missing data. NORM provides for the use of packet-level forward error correction (FEC) techniques for efficient multicast repair and OPTIONAL proactive transmission robustness [RFC3453]. FEC-based repair can be used to greatly reduce the quantity of reliable multicast repair requests and repair transmissions [MdpToolkit] in a NACK-oriented protocol. The principal factor in NORM scalability is the volume of feedback traffic generated by the receiver set to facilitate reliability and congestion control. NORM uses probabilistic suppression of redundant feedback based on exponentially distributed random backoff timers. The performance of this type of suppression relative to other techniques is described in [McastFeedback]. NORM dynamically measures the group's round-trip timing status to set its suppression and other protocol timers. This allows NORM to scale well while maintaining reliable data delivery transport with low latency relative to the network topology over which it is operating. Feedback messages can be either multicast to the group at large or sent via unicast routing to the sender. In the case of unicast feedback, the sender relays the feedback state to the group to facilitate feedback suppression. In typical Internet environments, the NORM protocol will readily scale to group sizes on the order of tens of thousands of receivers. A study of the quantity of feedback for this type of protocol is described in [NormFeedback]. NORM is able to operate with a smaller amount of feedback than a single TCP connection, even with relatively large numbers of receivers. Thus, depending upon the network topology, it is possible for NORM to scale to larger group sizes. With respect to computer resource usage, the
NORM protocol does not need state to be kept on all receivers in the group. NORM senders maintain state only for receivers providing explicit congestion control feedback. However, NORM receivers need to maintain state for each active sender. This can constrain the number of simultaneous senders in some uses of NORM. 1.4. Environmental Requirements and Considerations All of the environmental requirements and considerations that apply to the "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401], "Forward Error Correction (FEC) Building Block" [RFC5052], and "TCP-Friendly Multicast Congestion Control (TFMCC) Protocol Specification" [RFC4654] also apply to the NORM protocol. The NORM protocol SHALL be capable of operating in an end-to-end fashion with no assistance from intermediate systems beyond basic IP multicast group management, routing, and forwarding services. While the techniques utilized in NORM are principally applicable to flat, end-to-end IP multicast topologies, they could also be applied in the sub-levels of hierarchical (e.g., tree-based) multicast distribution if so desired. NORM can make use of reciprocal (among senders and receivers) multicast communication under the Any-Source Multicast (ASM) model defined in "Host Extensions for IP Multicasting" [RFC1112], but it SHALL also be capable of scalable operation in asymmetric topologies such as Source-Specific Multicast (SSM) [RFC4607] where only unicast routing service is available from the receivers to the sender(s). NORM is compatible with IPv4 and IPv6. Additionally, NORM can be used with networks employing Network Address Translation (NAT) provided that the NAT device supports IP multicast and/or can cache UDP traffic source port numbers for remapping feedback traffic from receivers to the sender(s). 2. Architecture Definition A NormSession is comprised of participants (NormNodes) acting as senders and/or receivers. NORM senders transmit data content in the form of NormObjects to the session destination address, and the NORM receivers attempt to reliably receive the transmitted content using negative acknowledgments to request repair. Each NormNode within a NormSession is assumed to have a preselected unique 32-bit identifier (NormNodeId). NormNodes MUST have uniquely assigned identifiers within a single NormSession to distinguish between multiple possible senders and to distinguish feedback information from different receivers. There are two reserved NormNodeId values. A value of 0x00000000 is considered an invalid NormNodeId (NORM_NODE_NONE), and a value of 0xffffffff is a "wild card" NormNodeId (NORM_NODE_ANY).
While the protocol does not preclude multiple sender nodes concurrently transmitting within the context of a single NORM session (i.e., many-to-many operation), any type of interactive coordination among NORM senders is assumed to be controlled by the application- or higher-protocol layer. There are some OPTIONAL mechanisms specified in this document that can be leveraged for such application-layer coordination. As previously noted, NORM allows for reliable transmission of three different basic types of data content. The first type is NORM_OBJECT_DATA, which is used for static, persistent blocks of data content maintained in the sender's application memory storage. The second type is NORM_OBJECT_FILE, which corresponds to data stored in the sender's non-volatile file system. The NORM_OBJECT_DATA and NORM_OBJECT_FILE types both represent NormObjects of finite but potentially very large size. The third type of data content is NORM_OBJECT_STREAM, which corresponds to an ongoing transmission of undefined length. This is analogous to the reliable stream service provided by TCP for unicast data transport. The format of the stream content is application-defined and can be "byte" or "message" oriented. The NORM protocol provides for "flushing" of the stream to expedite delivery or possibly enforce application message boundaries. NORM protocol implementations MAY offer either (or both) in-order delivery of the stream data to the receive application or out-of- order (more immediate) delivery of received segments of the stream to the receiver application. In either case, NORM sender and receiver implementations provide buffering to facilitate repair of the stream as it is transported. All NormObjects are logically segmented into FEC coding blocks and symbols for transmission by the sender. In NORM, a FEC encoding symbol directly corresponds to the payload of NORM_DATA messages or "segment". Note that when systematic FEC codes are used, the payload of NORM_DATA messages sent for the first portion of a FEC encoding block are source symbols (actual segments of original user data), while the remaining symbols for the block consist of parity symbols generated by FEC encoding. These parity symbols are generally sent in response to repair requests, but some number MAY be sent proactively at the end of each encoding block to increase the robustness of transmission. When non-systematic FEC codes are used, all symbols sent consist of FEC encoding parity content. In this case, the receiver needs to receive a sufficient number of symbols to reconstruct (via FEC decoding) the original user data for the given block. Transmitted NormObjects are temporarily, yet uniquely, identified within the NormSession context using the given sender's NormNodeId, NormInstanceId, and a temporary NormTransportId. Depending upon the
implementation, individual NORM senders can manage their NormInstanceIds independently, or a common NormInstanceId could be agreed upon for all participating nodes within a session, if needed, as a session identifier. NORM NormTransportId data content identifiers are sender-assigned and applicable and valid only during a NormObject's actual transport (i.e., for as long as the sender is transmitting and providing repair of the indicated NormObject). For a long-lived session, the NormTransportId field can wrap and previously used identifiers will be re-used. Note that globally unique identification of transported data content is not provided by NORM and, if necessary, is expected to be managed by the NORM application. The individual segments or symbols of the NormObject are further identified with FEC payload identifiers that include coding block and symbol identifiers. These are discussed in detail later in this document. 2.1. Protocol Operation Overview A NORM sender primarily generates messages of type NORM_DATA. These messages carry original data segments or FEC symbols and repair segments/symbols for the bulk data/file or stream NormObjects being transferred. By default, redundant FEC symbols are sent only in response to receiver repair requests (NACKs) and thus normally little or no additional transmission overhead is imposed due to FEC encoding. However, the NORM implementation MAY be configured to proactively transmit some amount of redundant FEC symbols along with the original content to potentially enhance performance (e.g., improved delay) at the cost of additional transmission overhead. This configuration is sensible for certain network conditions and can allow for robust, asymmetric multicast (e.g., unidirectional routing, satellite, cable) [FecHybrid] with reduced receiver feedback, or, in some cases, no feedback. A sender message of type NORM_INFO is also defined and is used to carry OPTIONAL out-of-band context information for a given transport object. A single NORM_INFO message can be associated with a NormObject. Because of its atomic nature, missing NORM_INFO messages can be NACKed and repaired with a slightly lower delay process than NORM's general FEC-encoded data content. The NORM_INFO message can serve special purposes for some bulk transfer, reliable multicast applications where receivers join the group mid-stream and need to ascertain contextual information on the current content being transmitted. The NACK process for NORM_INFO will be described later. When the NORM_INFO message type is used, its transmission SHOULD precede transmission of any NORM_DATA message for the associated NormObject. The sender also generates messages of type NORM_CMD to assist in
certain protocol operations such as congestion control, end-of- transmission flushing, group round-trip time (GRTT) estimation, receiver synchronization, and OPTIONAL positive acknowledgment requests or application-defined commands. The transmission of NORM_CMD messages from the sender is accomplished by one of three different procedures: single, best-effort unreliable transmission of the command; repeated redundant transmissions of the command; and positively acknowledged commands. The transmission technique used for a given command depends upon the function of the command. Several core commands are defined for basic protocol operation. Additionally, implementations MAY wish to consider providing the OPTIONAL application-defined commands that can take advantage of the transmission methodologies available for commands. This allows for application-level session management mechanisms that can make use of information available to the underlying NORM protocol engine (e.g., round-trip timing, transmission rate, etc.). A notable distinction between NORM_DATA message and some NORM_CMD message transmissions is that typically a receiver will need to allocate resources to manage reliable reception when NORM_DATA messages are received. However, some NORM_CMD messages are completely atomic and no specific reliability (buffering) state needs to be kept. Thus, for session management or other purposes, it is possible that even participants acting principally as data receivers MAY transmit NORM_CMD messages. However, it is RECOMMENDED that this is not done within the context of the NORM multicast session unless congestion control is addressed. For example, many receiver nodes transmitting NORM_CMD messages simultaneously can cause congestion for the destination(s). All sender transmissions are subject to rate control governed by a peak transmission rate set for each participant by the application. This can be used to limit the quantity of multicast data transmitted by the group. When NORM's congestion control algorithm is enabled, the rate for senders is automatically adjusted. In some networks, it is desirable to establish minimum and maximum bounds for the rate adjustment depending upon the application even when dynamic congestion control is enabled. However, in the case of the general Internet, congestion control policy SHALL be observed that is compatible with coexistent TCP flows. NORM receivers generate messages of type NORM_NACK or NORM_ACK in response to transmissions of data and commands from a sender. The NORM_NACK messages are generated to request repair of detected data transmission losses. Receivers generally detect losses by tracking the sequence of transmission from a sender. Sequencing information is embedded in the transmitted data packets and end-of-transmission commands from the sender. NORM_ACK messages are generated in response to certain commands transmitted by the sender. In the general (and most scalable) protocol mode, NORM_ACK messages are sent
only in response to congestion control commands from the sender. The feedback volume of these congestion control NORM_ACK messages is controlled using the same timer-based probabilistic suppression techniques as for NORM_NACK messages to avoid feedback implosion. In order to meet potential application requirements for positive acknowledgment from receivers, other NORM_ACK messages are defined and are available for use. 2.2. Protocol Building Blocks The operation of the NORM protocol is based primarily upon the concepts presented in the Multicast NACK Building Block [RFC5401] document. This includes the basic NORM architecture and the data transmission, repair, and feedback strategies discussed in that document. The reliable multicast building block approach, as described in "Reliable Multicast Transport Building Blocks for One- to-Many Bulk-Data Transfer" [RFC3048], is applied in creating the full NORM protocol instantiation. NORM also makes use of the parity- based encoding techniques for repair messaging and added transmission robustness as described in "The Use of Forward Error Correction (FEC) in Reliable Multicast" [RFC3453]. NORM uses the FEC Payload ID as specified by the FEC Building Block document [RFC5052]. Additionally, for congestion control, this document fully specifies a baseline congestion control mechanism (NORM-CC) based on the TCP- Friendly Multicast Congestion Control (TFMCC) scheme [TfmccPaper], [RFC4654]. 2.3. Design Trade-Offs While the various features of NORM provide some measure of general purpose utility, it is important to emphasize the understanding that "no one size fits all" in the reliable multicast transport arena. There are numerous engineering trade-offs involved in reliable multicast transport design and this necessitates an increased awareness of application and network architecture considerations. Performance requirements affecting design can include: group size, heterogeneity (e.g., capacity and/or delay), asymmetric delivery, data ordering, delivery delay, group dynamics, mobility, congestion control, and transport across low-capacity connections. NORM contains various parameters to accommodate many of these differing requirements. The NORM protocol and its mechanisms MAY be applied in multicast applications outside of bulk data transfer, but there is an assumed model of bulk transfer transport service that drives the trade-offs that determine the scalability and performance described in this document. The ability of NORM to provide reliable data delivery is also governed by any buffer constraints of the sender and receiver
applications. NORM protocol implementations SHOULD operate with the greatest efficiency and robustness possible within application- defined buffer constraints. Buffer requirements for reliability, as always, are a function of the delay-bandwidth product of the network topology. NORM performs best when allowed more buffering resources than typical point-to-point transport protocols. This is because NORM feedback suppression is based upon randomly delayed transmissions from the receiver set, rather than immediately transmitted feedback. There are definitive trade-offs between buffer utilization, group size scalability, and efficiency of performance. Large buffer sizes allow the NORM protocol to perform most efficiently in large delay-bandwidth topologies and allow for longer feedback suppression backoff timeouts. This yields improved group size scalability. NORM can operate with reduced buffering but at a cost of decreased efficiency (lower relative goodput) and reduced group size scalability. 3. Conformance Statement This RMT Protocol Instantiation document, in conjunction with the "Multicast Negative-Acknowledgment (NACK) Building Blocks" [RFC5401] and "Forward Error Correction (FEC) Building Block" [RFC5052] Building Blocks, completely specifies a working reliable multicast transport protocol that conforms to the requirements described in RFC 2357. This document specifies the following message types and mechanisms that are REQUIRED in complying NORM protocol implementations: +----------------------+--------------------------------------------+ | Message Type | Purpose | +----------------------+--------------------------------------------+ | NORM_DATA | Sender message for application data | | | transmission. Implementations MUST | | | support at least one of the | | | NORM_OBJECT_DATA, NORM_OBJECT_FILE, or | | | NORM_OBJECT_STREAM delivery services. The | | | use of the NORM FEC Object Transmission | | | Information header extension is OPTIONAL | | | with NORM_DATA messages. | | NORM_CMD(FLUSH) | Sender command to excite receivers for | | | repair requests in lieu of ongoing | | | NORM_DATA transmissions. Note the use of | | | the NORM_CMD(FLUSH) for positive | | | acknowledgment of data receipt is | | | OPTIONAL. |
| NORM_CMD(SQUELCH) | Sender command to advertise its current | | | valid repair window in response to invalid | | | requests for repair. | | NORM_CMD(REPAIR_ADV) | Sender command to advertise current repair | | | (and congestion control state) to group | | | when unicast feedback messages are | | | detected. Used to control/suppress | | | excessive receiver feedback in asymmetric | | | multicast topologies. | | NORM_CMD(CC) | Sender command used in collection of | | | round-trip timing and congestion control | | | status from group (this is OPTIONAL if | | | alternative congestion control mechanism | | | and round-trip timing collection is used). | | NORM_NACK | Receiver message used to request repair of | | | missing transmitted content. | | NORM_ACK | Receiver message used to proactively | | | provide feedback for congestion control | | | purposes. Also used with the OPTIONAL | | | NORM Positive Acknowledgment Process. | +----------------------+--------------------------------------------+ This document also describes the following message types and associated mechanisms that are OPTIONAL for complying NORM protocol implementations: +-----------------------+-------------------------------------------+ | Message Type | Purpose | +-----------------------+-------------------------------------------+ | NORM_INFO | Sender message for providing ancillary | | | context information associated with NORM | | | transport objects. The use of the NORM | | | FEC Object Transmission Information | | | header extension is OPTIONAL with | | | NORM_INFO messages. | | NORM_CMD(EOT) | Sender command to indicate it has reached | | | end-of-transmission and will no longer | | | respond to repair requests. | | NORM_CMD(ACK_REQ) | Sender command to support | | | application-defined, positively | | | acknowledged commands sent outside of the | | | context of the bulk data content being | | | transmitted. The NORM Positive | | | Acknowledgment Procedure associated with | | | this message type is OPTIONAL. |
| NORM_CMD(APPLICATION) | Sender command containing | | | application-defined commands sent outside | | | of the context of the bulk data content | | | being transmitted. | | NORM_REPORT | Optional message type reserved for | | | experimental implementations of the NORM | | | protocol. | +-----------------------+-------------------------------------------+ 4. Message Formats There are two primary classes of NORM messages (see Section 2.1): sender messages and receiver messages. NORM_CMD, NORM_INFO, and NORM_DATA message types are generated by senders of data content, and NORM_NACK and NORM_ACK messages generated by receivers within a NormSession. Sender messages SHALL be governed by congestion control for Internet use. For session management or other purposes, receivers can also employ NORM_CMD message transmissions. The principal rationale for distinguishing sender and receiver messages is that receivers will typically need to allocate resources to support reliable reception from sender(s) and NORM sender messages are subject to congestion control. NORM receivers MAY employ the NORM_CMD message type for application-defined purposes, but it is RECOMMENDED that congestion control and feedback implosion issues be addressed. Additionally, an auxiliary message type of NORM_REPORT is also provided for experimental purposes. This section describes the message formats used by the NORM protocol. These messages and their fields are referenced in the detailed functional description of the NORM protocol given in Section 5. Individual NORM messages are compatible with the Maximum Transmission Unit (MTU) limitations of encapsulating Internet protocols including IPv4, IPv6, and UDP. The current NORM protocol specification assumes UDP encapsulation and leverages the transport features of UDP. The NORM messages are independent of network addresses and can be used in IPv4 and IPv6 networks. 4.1. NORM Common Message Header and Extensions There are some common message fields contained in all NORM message types. Additionally, a header extension mechanism is defined to expand the functionality of the NORM protocol without revision to this document. All NORM protocol messages begin with a common header with information fields as follows:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |version| type | hdr_len | sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | source_id | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: NORM Common Message Header Format The "version" field is a 4-bit value indicating the protocol version number. NORM implementations SHOULD ignore received messages with version numbers different from their own. This number is intended to indicate and distinguish upgrades of the protocol that are non- interoperable. The NORM version number for this specification is 1. The message "type" field is a 4-bit value indicating the NORM protocol message type. These types are defined as follows: +------------------+------------------+ | Message | Value | +------------------+------------------+ | NORM_INFO | 1 | | NORM_DATA | 2 | | NORM_CMD | 3 | | NORM_NACK | 4 | | NORM_ACK | 5 | | NORM_REPORT | 6 | +------------------+------------------+ The 8-bit "hdr_len" field indicates the number of 32-bit words that comprise the given message's header portion. This is used to facilitate the addition of header extensions. The presence of header extensions is implied when the "hdr_len" value is greater than the base value for the given message "type". The "sequence" field is a 16-bit value that is set by the message originator. The "sequence" field serves two separate purposes, depending upon the message type: 1. NORM senders MUST set the "sequence" field of sender messages (NORM_INFO, NORM_DATA, and NORM_CMD) so that receivers can monitor the "sequence" value to maintain an estimate of packet loss that can be used for congestion control purposes (see Section 5.5.2 for a detailed description of NORM Congestion Control operation). A monotonically increasing sequence number space MUST be maintained to mark NORM sender messages in this way. Note that this "sequence" number is explicitly NOT used in
NORM as part of its reliability procedures. The NORM object and FEC payload identifiers are used to detect missing content for reliable transfer purposes. 2. NORM receivers SHOULD set the "sequence" field to support protection from message replay attacks of NORM_NACK or NORM_NACK messages. Note that, depending upon configuration, NORM feedback messages are sent to the session multicast address or the unicast address(es) of the active NORM sender(s). Thus, a separate, monotonically increasing sequence number space MUST be maintained for each destination address to which the NORM receiver is transmitting feedback messages. Note that these two separate purposes necessitate the maintenance of separate sequence spaces to support the functions described here. And, in the case of NORM receivers, additional sequence spaces are needed when feedback messages are sent to the sender unicast address(es) instead of the session address. The "source_id" field is a 32-bit value that uniquely identifies the node that sent the message within the context of a single NormSession. This value is termed the NORM node identifier (NormNodeId) and unique NormNodeIds MUST be assigned within a single NormSession. In some cases, use of the host IPv4 address or a hash of an address can suffice, but alternative methodologies for assignment and potential collision resolution of node identifiers within a multicast session SHOULD be considered. For example, the techniques for managing the 32-bit "synchronization source" (SSRC) identifiers defined in the Real-Time Protocol (RTP) specification [RFC3550] are applicable for use with NORM node identifiers when an ASM traffic model is observed. In most deployments of the NORM protocol to date, the NormNodeId assignments are administratively configured, and this form of NormNodeId assignment is RECOMMENDED for most purposes. NORM sender NormNodeId values MUST be unique within an ASM session so that NORM receiver feedback can be properly demultiplexed by senders, and NORM receiver NormNodeId values MUST also be unique for congestion control operation or when the OPTIONAL positive acknowledgment mechanism is used. NORM Header Extensions When header extensions are applied, they follow the message type's base header and precede any payload portion. There are two formats for header extensions, both of which begin with an 8-bit "het" (header extension type) field. One format is provided for variable- length extensions with "het" values in the range from 0 through 127. The other format is for fixed-length (one 32-bit word) extensions with "het" values in the range from 128 through 255.
For variable-length extensions, the value of the "hel" (header extension length) field is the length of the entire header extension, expressed in multiples of 32-bit words. The "hel" field MUST be present for variable-length extensions ("het" between 0 and 127) and MUST NOT be present for fixed-length extensions ("het" between 128 and 255). The formats of the variable-length and fixed-length header extensions are given, respectively, here: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het <=127 | hel | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Header Extension Content | | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: NORM Variable-Length Header Extension Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | het >=128 | reserved | Header Extension Content | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: NORM Fixed-Length (32-bit) Header Extension Format The "Header Extension Content" portion of the header extension is defined for each extension type. Some header extensions are defined within this document for NORM baseline FEC and congestion control operations.